A conceptual, linear reservoir runoff model to investigate melt season changes in cirque glacier hydrology

This paper presents a conceptual, linear reservoir runoff model and applies it to a small glacierized cirque basin in the French Pyrénées over the 1995 melt season. A series of modelling experiments are undertaken: (i) to explore the response of diurnal hydrograph form to seasonal changes in surface meltwater recharge and glacier storage and routing processes and (ii) to investigate the possible structure of the hydrological system of this remnant glacier. High resolution, spatially- and temporally distributed observations of snow and ice-melt (and precipitation records) are used to estimate bulk meltwater inputs, which feed into a lumped meltwater drainage model. Empirical hydrograph recession limb analysis provides a basis to identify the most likely ‘structure’ of the glacierʹs hydrological system. This structure is then represented in the model by two (‘fast’ and ‘slow’) linear reservoirs. Although fast reservoir storage coefficients show only a moderate decline (13.00–5.25 h), the proportion of bulk meltwater entering this reservoir increases as the glacier snowline retreats and the slow reservoir storage coefficient decreases (45.00–17.75 h); consequently modelled hydrographs become increasingly peaked over the ablation season. Later in the melt season, the drainage system is mathematically best represented as a single reservoir (with a storage coefficient of 6.00–8.25 h) due to meltwater production occurring mainly in the lower-mid ablation zone, reduction in the extent (capacity) of the slower storage areas, and/or integration of the slow and fast pathways. In terms of glacier hydrology, the modelling experiments suggest that the fast reservoir represents ice-melt draining into a semi-distributed system beneath the lower glacier and the slow reservoir represents a snowpack-fed distributed system below the upper glacier. The nature of storage and routing within the hydrological system and the degree to which these processes are significant in determining outflow from this vestigial glacier compared with larger glaciers raises some interesting scale-related issues. The paper also demonstrates the utility of simple conceptual modelling approaches for investigating glacier hydrological systems, in addition to their more traditional application in runoff forecasting.